The present invention relates to a package for packaging chips of ICs and other semiconductor devices. The present invention also relates to a process for producing such a package.
Recent advances in the technology of chips of ICs and other semiconductor devices are remarkable and active efforts are being made to increase the scale and the integrating density of such chips. One of the problems that accompany these efforts is how to effectively dissipate the heat generated in chips. Conventionally, alumina (Al.sub.2 O.sub.3) packages have been used with microelectronic chips but in order to achieve more efficient heat dissipation, various new types of packages have been proposed and reviewed. Among them are: (1) a ceramic package made of a material having good heat conductivity such as BeO, AlN or SiC; (2) an enamelled package consisting of an iron sheet with an enamel coating; (3) a package consisting of a metal substrate to which an insulator is bonded with an adhesive; (4) a package consisting of a metal substrate on which a ceramic powder is flame-sprayed; (5) a package consisting of a metal substrate on which a thin ceramic film is formed by a suitable method such as physical vapor deposition (PVD) or chemical vapor deposition (CVD); and (6) a package consisting of a metal substrate on the surface of which an insulating organic polymer layer is formed.
However, these packages have their own problems described below and are not completely satisfactory for use in practical applications.
(1) A ceramic package made of a good heat conductor such as BeO, AlN or SiC conducts 5-20 times as much heat as an alumina substrate but its manufacture requires a complicated process comprising purification of the starting powder, controlling the particle size of the powder, shaping the powder into a compact and sintering the compact. In addition, the need for performing sintering at elevated temperatures (1500.degree.-2000.degree. C.) involves such disadvantages as difficulty in preparing a large package, development of thermal distortion and high cost. Of the three materials mentioned above, BeO has the highest heat conductivity, but because of its toxic nature and high cost it can be used in only a very limited area.
(2) The manufacture of an enamelled package involves fusing enamel frit in the high temperature range of 650.degree.-800.degree. C., so that the resulting enamel layer as an insulator has a minimum thickness of 0.5 mm and reduces the heat conductivity of the package. A thin (.ltoreq.0.1 mm) enamel coat contains so many pinholes in its surface that its withstand voltage (dielectric strength) will fall to a commercially unacceptable level.
(3) A package consisting of a metal substrate to which an insulator such as alumina is bonded with an adhesive has also not been commercialized for several reasons such as an increased resistance to heat conduction in the adhesive layer and unevenness in the adhesive strength of various parts of the package.
(4) A package consisting of a metal substrate on which a ceramic powder is thermal-sprayed contains so many pinholes in the sprayed insulation layer that it does not have sufficient withstand voltage or the desired surface flatness of the insulation layer.
(5) A package consisting of a metal substrate on which a thin ceramic film is formed by a suitable method such as PVD or CVD requires heating at 500.degree. C. or higher in order to form the thin ceramic film and this causes such disadvantages as a decreased freedom in the choice of a suitable metal substrate and a reduction in the strength of the substrate. In addition, the adhesion between the thin ceramic film and the metal substrate is not strong and a great amount of unevenness is introduced in the film quality.
(6) A package having a thin organic polymer layer is also disadvantageous in that polymers have poor heat resistance and are not highly heat-conductive to serve as an efficient heat dissipater.
Metal layers that establish connection to the device or provide an interconnecting circuit are usually formed on the insulator (or insulating layer) of each of the packages described above. Conventionally, these metal layers are formed by techniques such as bonding with an adhesive, a thick-film process, PVD and CVD. A problem common to these techniques is that the metal layer does not adhere strongly to the insulation layer and separation between the two layers may readily occur if thermal stress is produced by for example, the soldering of lead wires. If one wants to provide stronger adhesion between the metal layer and the insulating layer, precise process control must be performed but then this results in high manufacturing cost.
Further, when a package is formed with a so-called thin film type laminated substrate 2 as shown in FIG. 5 in which ceramic thin film layers 6a, 6b, . . . and metal thin film layers 8a, 8b . . . are laminated on a base material 4, connection between different levels of thin metal layers must be established. Conventionally, a variety of processes such as forming through-holes in ceramic layers by plasma etching or with a micro drill, followed by printing a circuit pattern or depositing a thin metal layer and metalization, have been used. However, these processes have several problems to be solved such as the great length of time they require and the difficulty involved in achieving the required precision of working.